Charged particle beam apparatus

ABSTRACT

The present invention provides apparatuses to inspect small particles on the surface of a sample such as wafer and mask. The apparatuses provide both high detection efficiency and high throughput by forming Dark-field BSE images. The apparatuses can additionally inspect physical and electrical defects on the sample surface by form SE images and Bright-field BSE images simultaneously. The apparatuses can be designed to do single-beam or even multiple single-beam inspection for achieving a high throughput.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation application and claims priority toU.S. application Ser. No. 14/220,358 filed Mar. 20, 2014, which claimsthe benefit of priority to U.S. Provisional application No. 61/804,794entitled to Zhongwei Chen et al. filed Mar. 25, 2013 and entitled“Charged Particle Beam Apparatus”, the entire disclosures of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a charged particle beam apparatus whichemploys a scanning electron microscope (SEM) to inspect particles and/ordefects on a sample surface. More particularly, it relates to alow-voltage scanning electron microscope (LVSEM) for inspectingparticles and/or defects on surfaces of wafers or masks in semiconductormanufacturing industry.

2. Description of the Prior Art

In semiconductor manufacturing industry, sometime particles appear andremain on surfaces of masks and/or wafers during semiconductorfabrication process for some reasons, which impact the yield to a greatdegree. To monitor and therefore ensure the yield, optical apparatusesor called optical tools which are typically based on microscopy, havebeen employed to inspect particles after some fabrication processesbecause of their high inspection throughput and a good detectionefficiency. As integrations of IC chips are required higher and higher,critical dimensions of patterns on wafer and mask are shrunk, andconsequently smaller and smaller particles become killers in the yield.On development trends, optical tools are losing their abilities todetect killer particles due to their longer wavelengths relative toparticle dimensions.

Theoretically, an electron beam (e-beam) has a relatively shorterwavelength (such as 0.027 nm/2 keV) relative to particle dimensions(down to several nm), and therefore can provide higher detectionsensitivity for small particles than an optical beam. Higher detectionefficiency comes from higher detection sensitivity. Conventional e-beamapparatuses or called e-beam tools for inspecting defects on wafer/mask,which are based on Low-voltage Scanning Electron Microscopy (LVSEM), candirectly perform particle inspection. However they are always criticizedfor low throughput. A large beam current is necessary to get a highthroughput, but incurs strong Coulomb Effect which impacts the imageresolution and thereby reducing the sensitivity.

Accordingly, a new-type e-beam tool especially for small particleinspection, which can provide high detection efficiency and highinspection throughput, is needed. In addition, it will be advantageousif the tool can perform defect inspection as well.

SUMMARY OF THE INVENTION

The object of this invention is to provide an electron beam apparatusemploying LVSEM technology to inspect particles in a sample surface.Firstly, this invention employs the difference between the irregularscattering on particles and regular scattering on sample surface due toan illumination of a primary electron (PE) beam. By specificallyarranging illumination of PE beam and Collection of BackscatteredElectrons (BSEs) and Secondary Electrons (SEs) in a column of theapparatus, the apparatus can provide high detection sensitivity and highinspection throughput. Secondly, in the column, the objective lensand/or the condenser lens are compacted by using permanent magnets. Thecompact objective not only enables a favorable illumination of the PEbeam, but also reduces the size of the column. Then, this inventionconfigures the apparatus to accommodate multiple foregoing compactcolumns so as to inspect multiple areas of a sample simultaneously. So,this invention will especially benefit the particle inspection insemiconductor yield management.

Accordingly, the invention therefore provides a method for inspecting asurface of a sample, which comprises steps of providing a primaryelectron beam (PE beam) to illuminate and scan the surface of the sampleby oblique incidence, providing a first detector to detect backscatteredelectrons generated from the surface of the sample and traveling towardsan incidence side of the PE beam, and providing an electrode to collectsecondary electrons generated from the surface of the sample so as notto hit the first detector. The first detector has a through hole for thePE beam passing through. The electrode is placed close to theilluminated area on the surface of the sample. The inspecting method mayfurther comprise a step of providing a second detector to detectbackscattered electrons generated from the surface of the sample andtraveling towards a reflection side of the PE beam.

The present invention further provides a method for inspecting a surfaceof a sample, which comprises steps of providing a primary electron beam(PE beam) to illuminate and scan the surface of the sample by obliqueincidence, providing a first detector to detect backscattered electronsgenerated from the surface of the sample and traveling towards anincidence side of the PE beam, providing a grid electrode to attract andmake secondary electrons generated from the surface of the sample passthrough, and providing a second detector to detect the secondaryelectrons passing through the grid electrode. The first detector has athrough hole for the PE beam passing through. The grid electrode isplaced close to the illuminated area on the surface of the sample. Theinspecting method may further comprise a step of providing a thirddetector to detect backscattered electrons generated from the surface ofthe sample and traveling towards a reflection side of the PE beam.

The present invention therefore provides a device of detecting electronsgenerated from a surface of a sample, which comprises a first detectorhaving a through hole on an electron detection plane thereof, and afirst electrode beside the first detector and close to the samplesurface so as to attract and prevent secondary electrons generatedtherefrom (called as SEs) from hitting the first detector. The throughhole is for a primary electron beam (PE beam) passing through andilluminating the sample surface by oblique incidence, and the electrondetection plane is inclined towards the sample surface so as to collectbackscattered electrons generated therefrom by the PE beam and travelingtowards an incidence side thereof (called as Dark-field BSEs).

The first electrode can have a grid structure so as to attract and makethe SEs pass through. The detecting device may further comprise a seconddetector behind the first electrode so as to detect the SEstherethrough. The detecting device may further comprise a third detectoron an reflection side of the PE beam, wherein the third detector isinclined towards the sample surface so as to collect backscatteredelectrons generated therefrom by the PE beam and traveling towards thereflection side (called as Bright-field BSEs). The detecting device mayfurther comprise a second electrode in front of the electron detectionplane of the first detector, wherein the second electrode has a gridstructure so as to repel and prevent the SEs from passing through andmake the Bright-field BSEs pass through.

The present invention therefore provides a single-beam apparatus whichcomprises an electron source emitting primary electrons along an opticalaxis of the single-beam apparatus, a gun aperture plate below theelectron source and having at least one first opening, a condenser lensbelow the gun aperture plate and aligned with the optical axis, abeam-limit aperture plate below the condenser lens and having at leastone second opening, a magnetic objective lens below the beam-limitaperture plate and aligned with the optical axis, a first scanningdeflector inside a bore of the magnetic objective lens, a first detectorbelow the magnetic objective lens and having a through hole, anattraction electrode beside the first detector, and a sample stage belowthe first detector and attraction electrode and for supporting a sample.The primary electrons form a primary electron beam (PE beam). One of thefirst openings is aligned with the optical axis and limits a current ofthe PE beam to a first current value. One of the second openings isaligned with the optical axis and limits the current of the PE beam to asecond current value. The through hole is on an electron detection planeof the first detector and aligned with the optical axis so that the PEcan pass through. A being-inspected surface of the sample is upturned.An illumination angle formed between the optical axis and a normal ofthe being-inspected surface is larger than 0°. The condenser lens andmagnetic objective lens together focus the PE beam onto thatbeing-inspected surface, thereby forming illumination thereon. Theelectron detection plane of the first detector faces aslant thatbeing-inspected surface so as to collect backscattered electrons whichare generated therefrom by the PE beam and travel towards an incidenceside thereof (called as Dark-field BSEs). The attraction electrodeattracts secondary electrons generated from that being-inspected surfaceby the PE beam (called as SEs) so as to prevent said SEs from hittingthe first detector. The first scanning deflector deflects the PE beam soas to scan the being-inspected surface, thereby obtaining a Dark-fieldBSEs image by said first detector.

The single-beam apparatus may further comprise a second scanningdeflector above the first scanning deflector, wherein the first andsecond scanning deflectors together deflect the PE beam so as to scanthat being-inspected surface with smaller deflection aberrations.

The illumination angle is preferred equal to or larger than 45°. Theattraction electrode can have a grid structure so as to attract and makethe SEs pass therethrough. The single-beam apparatus may furthercomprise a second detector above the attraction electrode so as todetect the SEs therethrough, thereby obtaining a SEs image. Thesingle-beam apparatus may further comprise a third detector above thatbeing-inspected surface and on a reflection side of the PE beam so as tocollect backscattered electrons generated by the PE beam and travellingtowards the reflection side (called as Bright-field BSEs), therebyobtaining a Bright-field BSEs image.

The magnetic objective lens may have an upper pole-piece and a lowerpole-piece both forming an axial magnetic-circuit gap. The firstscanning deflector can be inside the axial magnetic-circuit gap so as togenerate small deflection aberrations. The magnetic objective lens maycomprise a permanent magnet and an excitation coil. The condenser lensmay be a magnetic lens which has an inner pole-piece and an outerpole-piece both forming a radial magnetic-circuit gap. The single-beamapparatus may further comprise an acceleration tube to establish ahigh-energy region which is around the optical axis, starting from orbelow the gun aperture plate and stop above the first scanningdeflector.

The first detector is preferred positively biased with respect to thesample so as to detect the Dark-field BSEs with high signal gain. Thesingle-beam apparatus may further comprise a shielding box with a gridfront plane to prevent an electric field generated by the first detectorfrom influencing the PE beam, wherein the grid front plane directlyfaces the electron detection plane of the first detector so that theDark-field BSEs can pass through and be detected.

The present invention therefore provides a multi-beam apparatus whichcomprises a sample stage for supporting a sample and making abeing-inspected surface thereof upturned, and a plurality of single-beamunits above that being-inspected surface. Each of said single-beam unitscomprises an electron source emitting primary electrons along an opticalaxis of the single-beam unit, a gun aperture plate below the electronsource and having at least one first opening, a condenser lens below thegun aperture plate and aligned with the optical axis, a beam-limitaperture plate below the condenser lens and having at least one secondopening, a magnetic objective lens below the beam-limit aperture plateand aligned with the optical axis, a first scanning deflector inside abore of the magnetic objective lens, a first detector below the magneticobjective lens and having a through hole, and an attraction electrodebeside the first detector and above that being-inspected surface. Theprimary electrons form a primary electron beam (PE beam). One of thefirst openings is aligned with the optical axis and limits a current ofthe PE beam to a first current value. One of the second openings isaligned with the optical axis and limits the current of the PE beam to asecond current value. The magnetic objective lens comprises anexcitation coil and a permanent magnet. The through hole of the firstdetector is on an electron detection plane of the first detector andaligned with the optical axis so that the PE can pass through. Anillumination angle formed between the optical axis and a normal of thatbeing-inspected surface is larger than 0°. The condenser lens andmagnetic objective lens together focus the PE beam onto thatbeing-inspected surface, thereby forming illumination thereon. Theelectron detection plane of the first detector faces aslant thatbeing-inspected surface so as to collect backscattered electrons whichare generated therefrom by the PE beam and travel towards an incidenceside thereof (called as Dark-field BSEs). The attraction electrodeattracts secondary electrons generated from that being-inspected surfaceby the PE beam (called as SEs) to prevent the SEs from hitting the firstdetector. The first scanning deflector deflects the PE beam so as toscan that being-inspected surface, thereby obtaining a Dark-field BSEsimage by the first detector. Therefore each of the single-beam unitsinspects an area on that being-inspected surface, and consequently theplurality of single-beam units inspects a plurality of correspondingareas on that being-inspected surface simultaneously or on a schedule.

Each single-beam unit may further comprise a second scanning deflectorabove the first scanning deflector, wherein the first and secondscanning deflectors together deflect the PE beam so as to scan thatbeing-inspected surface with smaller deflection aberrations. Theillumination angle of each single-beam unit is preferred equal to orlarger than 45°.

The attraction electrode of each single-beam unit may have a gridstructure so as to attract and make the SEs pass therethrough. Eachsingle-beam unit may further comprise a second detector above theattraction electrode so as to detect said SEs therethrough, therebyobtaining an SEs image. Each single-beam unit can further comprises athird detector above that being-inspected surface and on a reflectionside of the PE beam so as to collect backscattered electrons generatedby the PE beam and traveling towards that reflection side (called asBright-field BSEs), thereby obtaining a Bright-field BSEs image.

The magnetic objective lens of each single-beam unit may have an upperpole-piece and a lower pole-piece both forming an axial magnetic-circuitgap. The first scanning deflector of each single-beam unit can be insidethe axial magnetic-circuit gap to generate small deflection aberrations.

The condenser lens of single-beam unit can be a magnetic lens with apermanent magnet and an excitation coil. Each single-beam unit mayfurther comprise an acceleration tube to establish a high-energy regionwhich is around the optical axis, staring from or below the gun apertureplate and stop above the first scanning deflector.

The first detector of each single-beam unit may be positively biasedwith respect to the sample so as to detect the Dark-field BSEs with highsignal gain. Each single-beam unit may further comprise a shielding boxwith a grid front plane to prevent an electric field generated by thefirst detector from influencing the PE beam, wherein the grid frontplane directly faces the electron detection plane of the first detectorso that the Dark-field BSEs can pass through and be detected.

Other advantages of the present invention will become apparent from thefollowing description taken in conjunction with the accompanyingdrawings wherein are set forth, by way of illustration and example,certain embodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be readily understood by the followingdetailed description in conjunction with the accompanying drawings,wherein the same or like reference numerals designate the same or likestructural elements, and in which:

FIGS. 1A˜1C are schematic illustrations of a fundamental configurationfor particle inspection of a sample in accordance with one embodiment ofthe present invention.

FIGS. 2A and 2B show a simulation result of an example of theconfiguration in accordance with the embodiment of the present inventionshown in FIG. 1A.

FIG. 3 is a schematic illustration of a configuration for inspectingparticles on a sample surface in accordance with another embodiment ofthe present invention.

FIG. 4A is a schematic illustration of a configuration formultifunctional inspection of a sample in accordance another embodimentof the present invention.

FIG. 4B is a schematic illustration of a configuration formultifunctional inspection of a sample in accordance another embodimentof the present invention.

FIG. 4C is a schematic illustration of a configuration formultifunctional inspection of a sample in accordance another embodimentof the present invention.

FIG. 5A is a schematic illustration of an apparatus for particleinspection of a sample in accordance with another embodiment of thepresent invention.

FIG. 5B is a schematic illustration of an apparatus for particleinspection of a sample in accordance with another embodiment of thepresent invention.

FIG. 5C is a schematic illustration of an apparatus for particleinspection of a sample in accordance with another embodiment of thepresent invention.

FIG. 6 is a schematic illustration of a configuration of a detector forparticle inspection of a sample in accordance with another embodiment ofthe present invention.

FIGS. 7A and 7B are schematic illustrations of a multi-column apparatusfor particle or multifunctional inspection of a sample in accordancewith another embodiment of the present invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

Various example embodiments of the present invention will now bedescribed more fully with reference to the accompanying drawings inwhich some example embodiments of the invention are shown. Withoutlimiting the scope of the protection of the present invention, all thedescription and drawings of the embodiments will exemplarily be referredto an electron beam. However, the embodiments are not be used to limitthe present invention to specific charged particles.

In the drawings, relative dimensions of each component and among everycomponent may be exaggerated for clarity. Within the followingdescription of the drawings the same or like reference numbers refer tothe same or like components or entities, and only the differences withrespect to the individual embodiments are described.

Accordingly, while example embodiments of the invention are capable ofvarious modifications and alternative forms, embodiments thereof areshown by way of example in the drawings and will herein be described indetail. It should be understood, however, that there is no intent tolimit example embodiments of the invention to the particular formsdisclosed, but on the contrary, example embodiments of the invention areto cover all modifications, equivalents, and alternatives falling withinthe scope of the invention.

In this invention, particle means any kind of uninvited protrusions.

In this invention, X, Y and Z axes form Cartesian coordinate, and asample surface is on XOY plane.

In this invention, “axial” means “in the optical axis direction of anapparatus, column or a lens”, while “radial” means “in a directionperpendicular to the optical axis”.

As it is well known that when a PE beam impinges on a sample, secondaryelectrons (energy≤50 eV) and backscattered electrons (50 eV<energy≤PElanding energy) will be generated and emitted from the sample surface.For Secondary electron (SE) emission, the angular distribution conformsLambert's law (proportional to cos ϕ, where ϕ is emission angle relativeto the surface normal) irrespective of energy and incidence angle α(relative to the surface normal) of PE beam and sample material. The SEyield δ increases with increasing incidence angle, and thischaracteristic provides possibility to image topography of the samplesurface.

For Backscattered electron (BSE) emission, the backscattered coefficientη increase with increasing incidence angle α and atomic number, and thisenable the imaging of both topography and material contrast of thesample surface. The angular distribution depends on energy and incidenceangle α of PE beam and sample material, which generally consists of twoparts, a diffusely scattered part with Lambert' distribution and areflection-like part with emission maximum. In the range α=0°˜60°, thediffusely scattered part remain approximately constant, which is anadvantage for observation of material contrast. The reflection-likeemission maximum is getting obvious with increasing incidence angle,i.e. in the case of oblique incidence (α≥45°).

If there is a particle on a sample surface, the values of incidenceangle of a PE beam will be different when respectively hitting thesample surface and the particle due to the differences in normaldirection. This difference provides possibilities for detecting theparticle by material contrast and topography contrast. High contrast ofa detection signal results in high detection sensitivity for theparticle. To get high contrast, it is advantage to reduce the backgroundcomponent (due to the collection of the electrons emitted from thesample surface) and/or increase the feature component (due to thecollection of the electrons emitted from the particle). In terms of theforegoing characteristics, this invention provides a method to detectparticles on a sample surface by detecting Dark-field BSE image withhigh contrast, and one of its basic configurations is shown in FIGS. 1A,1B, 3 4A, 4B and 4C.

In FIG. 1A, a PE beam 1 passes through a hole of a detector 3 andilluminates a surface of a sample 2 by oblique incidence. Consequently,BSEs 11 and SEs 12 are generated from the illuminated area of the samplesurface. Most of BSEs 11 travel to the reflection side, and a very smallpart of BSEs 11 will travel to the incidence side and collected by thedetector 3. Most of SEs 12 will travel upwards. In order to make thedetector 3 not collecting SEs 12, an electrode 4 is placed above theilluminated area of the sample surface and slightly positively biasedwith respect to the sample 2 and the detector 3 to attract SEs 12 awayfrom hitting the detector 3.

For the sample surface having particles thereon, the PE beam 1 may hit aparticle as shown in FIG. 1B if the sample surface is scanned by someways such as scanning the PE beam 1 and/or moving the sample in thedirections parallel to the sample surface. In this case, most of BSEs 11generated from the illuminated area of the particle travel to theincidence side of the PE beam 1 and consequently are collected by thedetector 3, while SEs 12 will be attracted by the electrode 4 andconsequently not hit the detector 3.

FIG. 1C shows the detection signal S3 of the detector 3 when the samplesurface is scanned. The abscissa shows the geometric location on thesample surface and the ordinate shows the detection signal S3 of thedetector 3 in arbitrary unit. As the PE beam 1 approaches the particleposition P1, the detection signal S3 is getting strong from the lowervalue S3_0 to the higher value S3_1. Because the detector 3 only detectsa small part of the diffusely scattered BSEs from the sample surface anda large part of both the diffusely scattered BSEs and thereflection-like scattered BSEs from the particle, the value S3_0 is verysmall with respect to the value S3_1. Therefore the particle will beshown in the scanning image with a dark background and a high contrastwhich is shown in the equation (1). Accordingly, the scanning image is aDark-field BSE image.

$\begin{matrix}{{Contrast} = \frac{{S\; 3\_ 1} - {S\; 3\_ 0}}{{S\; 3\_ 1} + {S\; 3\_ 0}}} & (1)\end{matrix}$

FIGS. 2A and 2B show the trajectories of PE, SE and BSE and theequipotential lines of an example of the forgoing configuration,respectively in the meridional plane XOZ and the sagittal plane YOZ. Theincidence angle α of the PE beam 1, in this embodiment, is 60°, and thedistance is 20 mm from the detector 3 to the sample surface along theincidence direction of the PE beam 1. When the potentials V2 and V3 ofthe sample 2 and the detector 3 are equal and the potential V4 of theelectrode 4 is 40V higher than V2 and V3, more than 90% SEs areattracted to hit the electrode 4.

FIG. 3 shows another way to prevent SEs 12 from hitting the detector 3.A gird 5 is placed between the detector 3 and the sample and preferredto be close to the detector. The grid 5 is made of materials ofelectrical conductor and its potential V5 is slightly negatively biasedwith respect to the potential V2 of the sample 2. In this case, thepotential V4 of the electrode 4 can be equal to slightly higher than V2.

In the foregoing basic configurations, the SEs 12 and BSEs 11 on thereflection side can also be detected to form SE image and Bright-fieldBSE image. Although these two images are not advantageous for particleinspection because the particle will be shown in a bright background anda lower contrast, the Bright-field BSE image can show the materialcontrast on the sample surface and the SE image can show the topographyof the sample surface. Therefore, the separation of the Dark-field BSEimage, the Bright-field BSE image and the SE image providespossibilities for multifunctional inspections as well as particleinspection. FIGS. 4A˜4C show how to detect the SE image and theBright-field image in the basic configuration shown in FIG. 1A.

In FIG. 4A, the electrode 4 has a grid structure, and a SE detector 6 isplaced above the electrode 4. The potential V6 of the SE detector 6 ismuch higher than the potential V2 of the sample 2 to attract andaccelerate the SEs passing through the grid 4 so as to obtain a highgain on the SE detection. The grid 4 and the electrically-shieldingcover 7 enclose the SE detector 6 so as to avoid the electrical fieldthereof leaking out. The potential V7 of the electrically-shieldingcover 7 can be equal to V4 or V2. The combination of the grid 4, thedetector 6 and the cover 7 is called as SE detection unit. In FIG. 4B, aBright-field BSE detector 8 is further added and placed on thereflection side so as to detect BSEs 11 on the reflection side. Comparedwith FIG. 1A, in FIG. 4C only a Bright-field BSE detector 8 is added andplaced on the reflection side so as to detect BSEs 11 on the reflectionside. As shown in FIG. 3, a grid can be placed between the Bright-fielddetector 8 and the sample 2 so as to prevent SEs 12 from hitting thedetector 8 as well.

Besides the collection efficiency and purity of the signal electrons,the size, current and landing energy of the probe spot of the PE beam 1on the sample surface are the other factors significantly determiningthe contrast of each image mentioned above. A larger probe currentwithin a smaller spot size will be advantageous for obtaining a highercontrast. The backscattered coefficient η is approximately independentof the landing energy in the range 5˜100 keV. Below 5 keV, η decreasesfor heavy elements and increases for light elements with decrease in thelanding energy. Because most of materials used in semiconductormanufacturing are light elements, it is preferred to use ≤5 keV landingenergy, i.e. in LVSEM mode. For the applications which need the SE imagewhile inspecting particles, a lower landing energy such as <3 keV willbe better to get a high SE yield δ. This invention provides threeembodiments of LVSEM in terms of the foregoing considerations, as shownin FIGS. 5A˜5C. FIGS. 5A˜5C only show the basic detection for particleinspection. They can further comprise the SE detection unit and/or theBright-field BSE detector if necessary.

FIG. 5A shows an elementary embodiment which comprises the column 100,the dark-field BSE detector 112 and the SE-attraction electrode 113. ThePE beam 121 is emitted from an electron source 101 and travel downwardalong the optical axis 150 of the column 100. To realize an obliqueincidence of the PE beam 121 on the surface of the sample 114, theoptical axis 150 is set to form an angle≥60° with respect to the surfacenormal of the sample. The electron source 101 is set at a negativepotential Vc with respect to the ground potential, while the anode 102and the gun aperture 103 are at a potential Va higher than Vc and theground potential respectively. Therefore the PE beam 121 is acceleratedto a kinetic energy e·(Vc−Va) after passing through the gun aperture103. Then the PE beam 121 is slightly focused by the condenser lens 104and partially limited by the beam-limit aperture 105 to get a desiredprobe current on the sample 114. The condenser lens 104 can be amagnetic lens although it is shown as an electrostatic lens here. Nextthe PE beam 121 passes through the objective lens 111 and a scanningdeflector 110 and finally impinges on the surface of the sample 114. Theobjective lens 111 finely focuses the PE beam 121 to form a small probespot, while the deflector 110 deflects the PE beam 121 to scan thesample surface with the finely focused probe spot, thereby obtaining afield of view (FOV). The SE-attraction electrode 113 attracts SEs 122 soas to make the detector 112 only detect the dark-field BSEs 123.

Except the electron source 101, the anode 102 and the electrode 113, allthe other parts of the column 100 are set at the ground potential forthe sake of convenience in column manufacturing. In this case thelanding energy of the PE beam 121 is equal to e. Vc. If the detector 112needs to be set at a potential much higher than the ground potential forobtaining a higher signal gain, it can be covered by a shielding box asshown in FIG. 6. In FIG. 6, the shielding box 117 is set at the groundpotential and has a front grid 118 for the BSEs passing through. Thepotential V5 of the grid 118 can be equal to the ground potential or alight lower than the ground potential for repelling SE from hitting thedetector 112. Either or both of the gun aperture 103 and beam-limitaperture 105 can be a plate with one opening or a movable plate withseveral different-size openings for obtaining various desired values ofthe probe current and corresponding small spot sizes.

The objective lens 111 can be either an electrostatic lens or a magneticlens. However, a magnetic objective lens is preferred due to its smallaberrations. For a magnetic objective lens, its aberrations decreaseswith decrease in its working distance (WD, the axial distance betweenthe lower surface of the objective lens and the sample surface). Becauseof the oblique incidence, a short WD requires the objective lens 111 hasa small volume and a conical front end. Accordingly, the objective lens111 is proposed to have a permanent magnet 107 as well as an excitationcoil 108. The strong permanent magnet 107 provides a fixed magneticexcitation which takes the fixed and large part for the requiredmagnetic excitation range, and the coil 108 provides an adjustablemagnetic excitation to cover the rest small part. Because the coil 108only takes space much less than a conventional magnetic lens withoutpermanent magnet, the objective lens 111 can be constructed small involume. The upper pole-piece 109_1 and the lower pole-piece 109_2sandwiches the permanent magnet 107 and forms an axial magnetic-circuitgap close to the optical axis 150. The magnetic field leaked out throughthis magnetic-circuit gap focuses the PE beam 121 onto the samplesurface. The scanning deflector 110 can be either electrostatic ormagnetic. However, an electrostatic one is preferred due to its abilityto deflect the PE beam 121 with high speed. The scanning deflector 110is placed inside the magnetic-circuit gap between the upper and lowerpole-pieces so as to reduce the aberrations generated by the deflection.

During scanning, defocus of the PE beam 121 on the sample surface willappear because the sample surface is not perpendicular to the opticalaxis 150 in oblique incidence. The defocus can be dynamicallycompensated by adjusting the excitation current of the coil 108.Besides, the scanning deflector 110 can be a multiple lens which cangenerate a dipole field for deflection scanning and a round lens fieldfor the compensation of the defocus. In addition, the SE-attractionelectrode 113 will slightly divert the PE beam 121 as well as attractingSEs 122. The deviation will incur a position shift of the PE beam 121 onthe sample surface. The shift can also be compensated by the scanningdeflector 110.

FIG. 5B shows an advanced embodiment which can provide a larger probecurrent within a smaller spot size and over a larger FOV, in comparisonwith the foregoing elementary embodiment. For the sake of the clarity,the column is denoted as 200. In the column 200, at first, the magneticcondenser lens 104 is designed to have a radial magnetic-circuit gap sothat the electron source 101 can be deeply immersed in the magneticfield of the magnetic condenser lens 104. The deep immersion greatlyreduces the aberrations of the condenser lens 104, especially when thecondenser lens 104 works with a strong focus power to make a large PEcurrent passing through the beam-limit aperture 105. In addition,similar to the objective lens 111, the condenser lens 104 can have apermanent magnet 131 as well as an excitation coil 132. The strongpermanent magnet 131 provides a fixed magnetic excitation which takesthe fixed and large part for the required magnetic excitation range, andthe excitation coil 132 provides an adjustable magnetic excitation tocover the rest small part. Because the excitation coil 132 only takesspace much less than a conventional magnetic lens without permanentmagnet, the condenser lens 104 can be constructed small in volume. Theinner pole-piece 133 and the outer pole-piece 134 sandwiches thepermanent magnet 131 and forms the radial magnetic-circuit gap facingthe electron source 101. Secondly, one more scanning deflector 106 isadded and placed above the scanning deflector 110. These two scanningdeflectors together realize swing deflection (proposed in U.S. Pat. No.6,392,231) to further reduce off-axis aberrations due to the deflectionscanning. The effect can greatly increase the effective FOV.

FIG. 5C shows a more advanced embodiment which can provide a largerprobe current within a smaller spot size, in comparison with theforegoing embodiments. For the sake of the clarity, the column isdenoted as 300. In FIGS. 5A and 5B, from the gun aperture 103 to thesample 114, the PE beam 121 travels with the energy same as the landingenergy which is equal to e·Vc and ≤5 keV. If the PE beam 121 has a largecurrent, the Coulomb effect occurring on the way will obviously increasethe final spot size of the PE beam 121. In the column 300, to reduce theCoulomb effect, a high-energy region is established around the opticalaxis 150 and between the gun aperture and the scanning deflector 110 byplacing an acceleration tube 116 over there. The acceleration tube 116is set at a potential Ve much higher than the sample potential V2 whichis equal to the ground potential. Consequently, the PE beam 121 passesthrough the high-energy region with energy e·(Vc−Ve) much higher thanthe landing energy e·Vc. The higher the energy of the electron beam is,the weaker the Coulomb effect is. Although the upper end of theacceleration tube 116 is shown close to the gun aperture here, it can beconnected with the gun aperture, and thereby extending the high-energyregion upwards to the gun aperture.

In FIG. 5C, one more detector 115 is placed below the beam-limitaperture 105 to detect the BSEs passing through the hole of theDark-field BSEs detector 112 so as to increase the collection efficiencyof the signal electrons. The detector 115 has a hole for the PE beam 121passing through. The acceleration tube 116 has one or more side openingsdesigned for installing and replacing the beam limit aperture 105 andthe detector 115 when operating routine maintenance.

Although the methods used in FIGS. 5B and 5C enable the columns togenerate a smaller spot size with a large probe current, the inspectionthroughput by a single beam may be not sufficient for some applicationsin mass production. In those cases, using multiple beams to doinspection is a powerful solution. For structuring a multi-beamapparatus with multiple single-beam columns, the number of electronbeams available for a wafer or a mask is limited by the spatial intervalrequired to physically accommodate two adjacent single-beam columns inparallel. The column embodiments shown in FIGS. 5A˜5C are small involume due to the employment of permanent magnet in the magneticobjective lens. This advantage can greatly reduce the spatial interval,thereby increasing the number of electron beams available for a wafer ora mask. FIG. 7A and FIG. 7B are respectively top and side views of amulti-beam apparatus 1000 with twelve units 11˜16 and 21˜26 forinspecting particles on a wafer/mask respectively and simultaneously.Each of the twelve units comprises a column as shown in FIGS. 5A˜5C anda detection configuration as shown in FIGS. 1A, 1B, 3 4A,4B and 4C. Theunits 11˜16 are placed on the left side and form a left-side group,while the units 21˜26 are placed on the right side and form a right-sidegroup. Each unit of the left-side group illuminates the surface of thesample 10 in oblique incidence from the left side, such as the PE beam16-1 of the unit 16 shown in FIG. 7B. Accordingly, each unit of theright-side group illuminates the surface of the sample 10 in obliqueincidence from the right side, such as the PE beam 26-1 of the unit 26shown in FIG. 7B.

In summary this invention provides methods to construct e-beamapparatuses for inspecting small particles on the surface of a samplesuch as wafer and mask. The apparatuses fundamentally provide both highdetection efficiency and high throughput by forming Dark-field BSEimage. Besides, they are able to form SE image and Bright-field BSEimage simultaneously to realize additional inspection functions such asinspecting physical and electrical defects on the sample surface aswell. The e-beam apparatus can comprise only one single-beam unit to dosingle-beam inspection or multiple single-beam units to form multi-beaminspection for achieving high throughput. In the column of a single-beamunit, the objective lens or both of the objective lens and the condenserlens is/are compacted by using permanent magnet inside. The compactstructure not only enables the favorable oblique incidence of the PEbeam, but also increases the number of single-beams available for awafer or a mask. This invention will especially benefit the particleinspection in semiconductor yield management.

Although the present invention has been described in accordance with theembodiments shown, one of ordinary skill in the art will readilyrecognize that there could be variations to the embodiments and thosevariations would be within the spirit and scope of the presentinvention. Accordingly, many modifications may be made by one ofordinary skill in the art without departing from the spirit and scope ofthe invention as limited only by the appended claims.

What is claimed is:
 1. A method for inspecting a surface of a sample,comprising: providing an incidence side detector with a through hole;scanning said surface at an oblique angle of incidence with a primaryelectron beam passing through said through hole; detecting backscatteredelectrons generated from said scanning and traveling towards anincidence side of said primary electron beam with said incidence sidedetector; positioning an electrode adjacent to a path of said primaryelectron beam, along a path of said backscattered electrons,intersecting a first virtual axis, wherein said first virtual axis isperpendicular to a sample plane and intersecting an illuminated area ofsaid surface, and closer than said incidence side detector to anilluminated area of said surface; attracting secondary electronsgenerated from said surface with said electrode to prevent saidsecondary electrons from otherwise hitting said incidence side detector;and generating a dark-field backscattered electron image from saidbackscattered electrons detected by said incidence side detector.
 2. Themethod according to claim 1, further comprising detecting backscatteredelectrons generated from said scanning and traveling towards areflection side from said primary electron beam with a reflection sidedetector.
 3. The method claim 1, further comprising providing asecondary electron detector, wherein said electrode is a grid electrode,and said secondary electron detector detects said secondary electronspassing through said grid electrode.
 4. The method according to claim 3,further comprising detecting backscattered electrons generated from saidscanning and traveling towards a reflection side from said primaryelectron beam with a reflection side detector.
 5. The method of claim 1,wherein said incidence side detector is disposed between an objectivelens of said primary electron beam and said surface.
 6. The methodaccording to claim 5, further comprising detecting backscatteredelectrons generated from said scanning and traveling towards areflection side from said primary electron beam with a reflection sidedetector.
 7. The method according to claim 3 wherein said incidence sidedetector is disposed between an objective lens of said primary electronbeam and said surface.
 8. The method according to claim 7, furthercomprising detecting backscattered electrons generated from saidscanning and traveling towards a reflection side from said primaryelectron beam with a reflection side detector.
 9. The method accordingto claim 1, wherein said sample is disposed on said sample plane, andsaid incidence side detector is disposed on an electron detection planegreater than or equal to 45 degrees to said sample plane.
 10. The methodaccording to claim 2, wherein said sample is disposed on said sampleplane, and said incidence side detector is disposed on an electrondetection plane greater than or equal to 45 degrees to said sampleplane.
 11. The method according to claim 3, wherein said sample isdisposed on said sample plane, and said incidence side detector isdisposed on an electron detection plane greater than or equal to 45degrees to said sample plane.
 12. The method according to claim 4,wherein said sample is disposed on said sample plane, and said incidenceside detector is disposed on an electron detection plane greater than orequal to 45 degrees to said sample plane.
 13. The method according toclaim 5, wherein said sample is disposed on said sample plane, and saidincidence side detector is disposed on an electron detection planegreater than or equal to 45 degrees to said sample plane.
 14. The methodaccording to claim 6, wherein said sample is disposed on said sampleplane, and said incidence side detector is disposed on an electrondetection plane greater than or equal to 45 degrees to said sampleplane.
 15. The method according to claim 7, wherein said sample isdisposed on said sample plane, and said incidence side detector isdisposed on an electron detection plane greater than or equal to 45degrees to said sample plane.
 16. The method according to claim 8,wherein said sample is disposed on said sample plane, and said incidenceside detector is disposed on an electron detection plane greater than orequal to 45 degrees to said sample plane.
 17. A method for inspecting asurface of a sample, comprising: providing an inspection tool using acharged particle beam, wherein a first detector, with a through hole forpassing the charged particle beam, is located between the inspectiontool and the sample; scanning the surface at an oblique angle ofincidence with the charged particle beam; attracting secondary electronsemanating from the surface by using an electrode positioned closer tothe surface than said first detector; and detecting, by the firstdetector, backscattered charged particles bouncing back from the surfaceand along a direction of the oblique angle.
 18. The method according toclaim 17, wherein in said detecting step, the detected backscatteredcharged particles are close to zero when the surface is flat, and thebackscattered charged particles are detected when the surface is notflat, thereby obtaining a dark field image.